Older, more established FCC regeneration units are operated in an incomplete mode of combustion defined by a CO content of from 1 to 6 volume percent. Substantially complete combustion of coke on an FCC molecular sieve catalyst is disclosed in Bertolacini et al U.S. Pat. No. 4,435,282. This exemplifies the type of system where the quantity of CO content is usually less than 500 ppm. The gaseous effluent from such a regeneration unit has a low CO content and a high CO.sub.2 content.
A multi-stage process to reduce NO.sub.x in a flue gas is disclosed in Herbst et al U.S. Pat. No. 4,868,144 (1989) in which NO.sub.x formed in a multiple stage regeneration is converted to nitrogen prior to discharge by operating the downstream end of each regeneration stage at oxygen-lean conditions. In effect, the oxygen concentration in these ranges is sufficiently low to shift the equilibrium to convert NO.sub.x produced during the regeneration to nitrogen. See also U.S. Pat. No. 4,853,187 (1989). A two-stage method of regeneration is disclosed in Yan et al, U.S. Pat. No. 4,851,374 (1989) in which a second fluidizing gas such as air is added to the second stage to regenerate the catalyst. Such addition provides rapid regeneration of the catalyst, suppression of SO.sub.x in the presence of a NO.sub.x transfer agent and substantially complete burning of CO to CO.sub.2. The gaseous environment of the second stage, however, favors excessive concentration of NO.sub.x. Since most of the nitrogen compounds are destroyed in the first stage, a lower overall content of CO, NO.sub.x and SO.sub.x are achieved through the two-stage regeneration system. A three-stage catalyst regeneration system is provided in Scott U.S. Pat. No. 4,325,833 (1982) where the problem is recognized that some types of cracking operations increase NO.sub.x content of the flue gas. Additional oxygen is added to the regeneration system to convert coke and oxygen to carbon monoxide resulting in a substantially oxygen-free atmosphere. Nitrogen oxides contained in the regeneration gas are reacted to form free nitrogen in the oxygen-free atmosphere so that the amount of nitrogen oxide in the regeneration gas is substantially reduced.
Finally, in Johnson et al, U. S. Pat. No. 4,744,962 a problem of NO.sub.x concentration in an off gas from an incomplete mode of regeneration is solved by the addition of NO.sub.x itself to either the regeneration off gas or to the regenerator itself. The addition of the NO.sub.x reduces the amount of ammonia in the off gas stream thereby changing the feed to the CO combustor to eliminate the production of NO.sub.x in the CO combustor.